1. Field of the Invention
The present invention relates to a method for preparing a thermosetting powder coating material as well as to powder coating materials per se.
2. Description of Related Art
Heretofore, a thermosetting powder coating material is prepared by dryblending a base resin, a crosslinking agent, and if necessary, a pigment and an addition agent using a mixer, then cooling and grinding them. However, this preparation method has some problems. For example, it is generally a complicated process because the preparation includes a mixed melting with heating after dryblending, which can easily lead to topical heating because temperature control is difficult when a mixed melting and heating are employed. Thus, due to the fear of increased viscosity and gelation, a low temperature curing type coating material generally is difficult to prepare. In addition, such processes also have problems relating to insufficient dispersion of pigment and crosslinking agent. Thus, the final finished appearance and curability are generally not as desired when a dispersion of pigment and a crosslinking agent is accomplished by mixed melting in the two axes extruder. Moreover, the above-mentioned method for preparing a powder coating material is generally not appropriate for preparing a powder coating material when many colors are required for an enamel system coating material, because cleaning the two axes extruder and the like often takes a lot of time.
Furthermore, the thermosetting powder coating material is often applied to form a thick film in contrast to a solvent type coating material. As such, it often volatilizes only to a small extent upon curing, so that internal stress is easily generated. Thus a coating film prepared in this way inconveniently tends to peel off by water and/or heat. Additionally, in the thermosetting powder coating material, a relatively hard compound is used in large quantities to provide antiblocking properties. Therefore, inconveniently, the flexibility of the coating film is often insufficient and antichipping properties can be poor.
As such, providing a dispersion of fine particles in the powder coating material can be carried out to relieve the internal stress and to improve the antichipping properties. However, when the fine particles of a synthetic resin having a low glass transition temperature dispersed in water are heated and dried, these particles become molten and their re-dispersion is almost impossible. Furthermore, even if these particles are freeze-dried, aggregates of several microns are formed, and even if these particles are dispersed at the time of the melting and kneading, they cannot be dispersed as primary particles. As such, the use of fine resin particles alone generally cannot contribute to the improvement of the physical properties of the coating film.